First press the more button in the name filter panel. Another window comes up. add *.* in the name exclusion panel.
Make sure the pattern type in the panel at the top indicates 'unix wildcard' - this is the default pattern type for the name filter.
Press OK.
This will tell the name filter to omit any files with a '.' in the name.

Back on the main window, enter the following into the custom renaming creator (do not enter the single quotes).

Row: 1
Search: '(.*)'
Replace: '\1.wav'

This will tell the renamer to take all of the current name and add '.wav'.
Press Scan.